This invention relates to the testing of magnetic bubble domain chip modules, and, in particular, to a method and apparatus for testing and setting the magnetic field strength of the permanent magnets in a completed magnetic bubble module so as to adjust its bias field to a precise level during the final manufacturing process.
Magnetic bubble devices require a magnetic field, usually called the bias field, perpendicular to the plane of the active surface of the chip. This field forms the magnetic bubbles on the substrate, controls the size of the bubbles, and keeps them in a non-volatile state. The strength of the bias field is critical, and slight deviations in this field strength from the optimal value strongly affect the operation of the chip. This bias field may be supplied by a structure employing permanent magnets and metal alloy plates for improving the uniformity of the magnetic field across the surface of the chip. Since a precise magnetic field is required, and the strength of this field could be adversely affected by stray magnetic fields in the environment in which the chip is to be operated, the magnetic bubble domain module is typically provided with an enclosure consisting of a magnetic shielding material which shunts these stray fields. The magnetic bubble chip, appendant assemblies, and magnetic bias field structure are then encapsulated with an insulating material which fills the enclosure of shielding material. It is thus quite difficult to precisely reset or adjust the bias field after the enclosure of shielding material has been incorporated into the module and the encapsulating insulating material has been introduced in the enclosure without resorting to disassembly.
Small deviations which occur during fabrication and assembly of the magnetic bubble domain chips and devices causes each device to require a slightly different bias field strength. One method of supplying the bias field involves testing the magnetic bubble domain chip separately, testing the bias field structure separately, and matching the magnetic bubble domain chip and bias field structure before the final manufacturing process in which the two are joined. By way of illustration, using this method, a magnetic bubble domain chip which operates best at a 108 Oe. is placed in a bin with other 108 Oe. chips, and a bias field structure magnetized to 108 Oe. is placed in a bin with other 108 Oe. bias field structures. The elements in the bins are matched and sent into the final manufacturing process in which the module is completed. One problem with this method is its inaccuracy, since the final packaging steps will produce deviations which will affect the precise relationship between the bias field and the chip supposedly insured by the matching process. It is also expensive and time consuming, and does not readily lend itself to assembly line volume production procedures.
Substrate impurities and minor deviations in fabrication procedures commonly occurring in the production of magnetic bubble domain chips may cause portions of the chip to be non-functional. Because the number of non-functional areas of the chips is a function of the bias field strength, a non-optimal bias field will result in even more defective areas on the chip. It would thus be desirable to set the bias field at such a value during the manufacturing process so as to minimize the number of non-functional areas on the chip.